The back-and-forth battle that is redistricting swung in Republicans’ favor when a redistricting commission in New Jersey picked a map favored by the GOP over the holiday weekend.

The bipartisan commission’s tie-breaking chairman on Friday picked a map that is pitting Democratic Reps. Steve Rothman and Bill Pascrell against each other and should help Republicans keep their most vulnerable incumbents in Congress.

The news, combined with recent developments in two bigger states — Texas and Pennsylvania — means Republicans have regained their advantage when it comes to creating districts that they will be favored to win in the 2012 election.

The newly updated Washington Post Redistricting Scorecard shows that, with 27 of 43 states having finished their congressional redistricting process (seven other states have just one district), the GOP has a one-seat edge over Democrats.

And the difference, at least for right now, is New Jersey.

With the state losing one of its 13 seats, there were many theories and ideas about which two incumbents would be drawn together (The Fix broached several of them back in April).

In the end, Rothman’s district was the one that got dismantled. His home was drawn into the conservative-leaning district held by Rep. Scott Garrett, but most of his district was combined with Pascrell’s. Rothman hasn’t publicly announced his plans, but the New Jersey Star-Ledger reported late Monday that he is telling people he will run against Pascrell rather than Garrett.

That’s probably the best move, politically speaking, and Democrats have the commission to thank for it.

Garrett’s 5th district isn’t unwinnable for Democrats, and in fact, it became more Democratic as a result of redistricting. While it’s currently a district that went 54 percent for Sen. John McCain (R-Ariz.) in the 2008 presidential race, it will become a 51 percent McCain district.

But it’s almost all Garrett’s territory and only takes in a small chunk of Rothman’s 9th district — enough to move Rothman’s home into the district.

The combined Rothman/Pascrell 9th district, meanwhile, actually has more of Rothman’s current 9th district than Pascrell’s 8th district. Rothman will reportedly move to Englewood so that he lives in the district. In that fight, he appears to have something closer to a 50-50 shot of returning to Congress.

But it also means Garrett dodged a tough incumbent-versus-incumbent race and that Democrats are down one seat.

But that wasn’t the only good news for the GOP. Its two most vulnerable members — freshman Rep. Jon Runyan and Rep. Leonard Lance — both got some help, and especially Lance.

Runyan’s 3rd district, which went 47 percent for McCain, got about one point better for the GOP and leans slightly Republican.

Lance’s 7th district, meanwhile, went from a district that went 50 percent for McCain to one that would go about 52 percent for him.

GOP Rep. Frank LoBiondo, who currently represents the swing 2nd district, also saw his district get a little bit better for the GOP. On paper, his district remains the most competitive in the state, but he has not been seriously challenged in recent years, and the seat is probably safe as long as he is there.

Democrats also got a little help with some of their more vulnerable members, with both Reps. Rush Holt and Frank Pallone getting safer districts (Holt’s got much safer). But the GOP wasn’t likely going to be able to target those two anyway.

In the end, the GOP will have a good shot at holding half of the 12 congressional districts in an otherwise blue-leaning state. But it may have to defend a few of its seats in the coming decade.

Factoring New Jersey into our Redistricting Scorecard’s projections, we now show Republicans are set to lose two seats and Democrats are set to lose three. (A few states that are gaining seats have yet to finish their redistricting process, so both parties are currently losing seats.)

The two big questions down the stretch are what happens in Texas and Florida. In both states, it appears that the new seats (four in Texas and two in Florida) could be split between the two parties. That would mean that the two sides would have essentially fought to a draw when it comes to getting new seats they can win during the 2011-2012 redistricting session.

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